Histoplasma capsulatum is an important endemic, intracellular fungal pathogen that can cause life threatening disseminated disease, particularly in individuals with impaired immunity. Despite the use of potent anti-fungal medications significant morbidity and mortality from histoplasmosis continues to occur, and current treatment regimens frequently fail to eradicate the organism in immuno-compromised individuals, particularly those with AIDS. Recent data from intracellular pathogens (i.e. Mycobacterium tuberculosis, Listeria monocytogenes) and other fungi (i.e. Cryptococcus neoformans, Candida albicans) demonstrates that specific antibody can improve the ability of the host to combat these infecting microbes. I have identified three murine monoclonal antibodies (mAb) that bind to the H. capsulatum yeast cell surface. Administration of the mAbs reduced fungal burden, decreased pulmonary inflammation, and prolonged survival in a murine infection model. MAb treatment was associated with enhanced production of lL-4, IL-6, and IFN- in the lungs of infected mice. The mAbs increased phagocytosis of yeast by macrophage via interactions dependent on CR3. Growth of mAb-opsonized H. capsulatum was inhibited after ingestion and killing of intracellular fungus occurred. The H. capsulatum-binding mAbs interact with a histone-like protein (histone H2B) expressed on the cell surface of the fungus. The goals of this application are to define the mechanism of antibody action against this fungus, to characterize the protective epitope on the antigen, and to use the protein in the generation of new mAbs and in vaccine development. Four specific Aims are proposed: 1. To map the epitope of mAb 9C7 and generate additional mAbs to H. capsulatum. 2. To investigate the mechanism(s) of antibody-mediated protection against H. capsulatum in vivo. 3. To investigate the mechanism(s) of antibody-mediated protection against H. capsulatum in vitro. 4. To determine the efficacy of the histone H2B and the mAb 9C7 epitope as vaccines. These studies are expected to lead to insights into H. capsulatum pathogenesis and initiate the development of new therapeutic and diagnostic reagents.